The present invention relates generally to control systems for motor starters, and more particularly to a method and apparatus to compensate a motor starter for increases in temperature.
Many electrical devices use one form or another for thermal protection. One of the most common forms of temperature protection includes the use of thermistors, which are heat sensitive resistors that change resistive value with temperature change. Typically, the thermistor is connected to an electronic monitoring circuit which is set to react to a predetermined resistance value. When that resistance value is reached, the electronic monitoring circuit disconnects, or connects, the temperature protection circuits, which then turn the device off. Most electrical devices use a single thermistor for protection. Therefore, the level of protection provided is solely dependent on the location of that single thermistor. In other words, the thermal protection in reality protects only one small portion of the device. In larger devices, many components can be damaged before the thermal protection turns off the device. Another problem with such systems is that they provide little warning or assistance in diagnosing the cause of shutting down the device. Further, in some applications or processes, it is critical that the electrical device not be shutdown. In such processes, it would be desirable to have some indication of an overheating condition while keeping the process running.
Another common method of thermal protection is the use of a bimetallic element or disk mounted within the device to open or close a circuit. Such devices change shape with heat due to a differential thermal expansion between the two metals that form the physical configuration. As the device changes shape, it exerts a physical force on a switch, or on a set of contacts, to change the state of an electrical circuit. That is, a normally opened circuit, for example, will close to activate a temperature protection circuit when the bi-metal strip deforms to a deflection point that corresponds to a temperature indicating an overheating condition. This form of thermal protection requires calibration by hand-bending or tweaking for each particular device. Further, accuracy is suspect after field adjustments are made and such bi-metal devices typically require invasive connections between components, thereby reducing manufactureability, increasing the cost of the component, and increasing the overall size of the electrical equipment.
An electrical device with thermal compensation adjusts the starting characteristics of the controller to compensate for the additional ambient temperature. This compensation adjusts or increases the output of the device to compensate for the increasing internal resistance of the device caused by heating. Typically, such temperature compensation requires the use of mechanical components, such as the aforementioned bimetallic elements to sense the temperature and adjust the output accordingly. Since thermal compensation ranges are relatively small, several device configurations are required in order to cover an entire product line. Further, thermal compensation adds additional cost to the manufacturing of the device because of the additional components.
It would be desirable to have a thermal compensation and protection scheme that eliminates the need of these additional components to avoid the need for having variations of the product for different temperature ranges.